JPH05340807A - Infrared detector - Google Patents

Abstract

PURPOSE:To stabilize the characteristics of an infrared detector and, at the same time, to improve the noise resistance of the detector and reduce the cost of the detector. CONSTITUTION:A power supply voltage is supplied to a pyroelectric infrared detection element 40 from a constant-voltage circuit 44 through a resistor 42 by using the FET 14 of the element 40 for source earthing. Because of the amplifying action of the FET 14, the amplification degree of an AC amplifier circuit 46 and the resistance of a feedback resistor 28 can be reduced and the stability and noise resistance of the circuit 46 can be improved. In addition, the cost of this infrared detector can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared detecting device using a pyroelectric infrared detecting element.

[0002]

2. Description of the Related Art A pyroelectric infrared detecting element is a differential type infrared detecting element, which detects a change in the incident amount of infrared rays and outputs an electric signal. Pyroelectric infrared detectors are infrared detectors that are highly sensitive, inexpensive, and capable of operating at room temperature, and are therefore widely used as human body detectors in the fields of crime prevention equipment and home appliances. Further, since the pyroelectric infrared detection element is a differential type, it is possible to detect a moving human body with a relatively simple circuit configuration.

As shown in FIG. 5, the pyroelectric infrared detecting element has a pyroelectric body 10 for infrared detection. The two pyroelectric bodies 10 are connected in series and generate an electric charge according to the intensity change of the incident infrared rays. This electric charge is converted into a voltage by the resistance element 12 connected in series with the pyroelectric body 10. One end of the resistance element 12 is connected to the gate of the FET 14, and the FET 14 is
Impedance conversion of the voltage signal generated at both ends of the.
A pyroelectric infrared detection element 16 is composed of the pyroelectric body 10, the resistance element 12, and the FET 14. Although not shown, the pyroelectric infrared detection element 16 has a substrate on which the pyroelectric body 10, the resistance element 12, the FET 14 and the like are mounted, and these are for selectively transmitting infrared rays. It is housed in a housing having a window.

The pyroelectric infrared detection element 16 is connected to an AC amplification circuit 18. The AC amplifier circuit 18 has an OP amplifier 20 as shown in FIG. The drain of the FET 14 is connected to the power supply, and the source of the FET 14 is connected to the AC amplification circuit 18. A resistor 22 is connected between the sources of the FET 14 and the ground, and a signal appearing at the source is input to the inverting input terminal of the OP amplifier 20 via the capacitor 24 and the input resistor 26. In this way, FET
Since 14 is used as a source follower, the source potential changes according to the change of the gate potential. The source potential does not change depending on the drain potential. The inverting input terminal of the OP amplifier 20 has a resistor 22 and a capacitor 24.
Since the differentiating circuit composed of is connected, this source potential is differentiated by the differentiating circuit and amplified by the OP amplifier 20. The output of the OP amplifier 20 is fed back to the inverting input terminal by the resistor 28 and the capacitor 30, and the amplification gain is set by the resistor 28.

FIG. 6 shows the entire circuit configuration of this conventional example. As shown in this figure, the pyroelectric infrared detection element 16 receives the infrared light incident through the window 34 provided in the housing 32, and outputs a voltage according to the change in the infrared incident amount. This voltage is amplified by about 60 to 70 dB by the AC amplification circuit 18 having the configuration shown in FIG. 5, and compared with a predetermined reference voltage by the comparison circuit 36 in the subsequent stage. When the output of the AC amplification circuit 18 is higher than the reference voltage, it can be considered that the moving human body is detected by the pyroelectric infrared detection element 16, and accordingly, the timer circuit 38 is activated for a certain period of time. The width detection pulse is output.

As described above, by using the pyroelectric infrared detecting element 16, the moving human body can be suitably detected, and the detecting device can be constructed relatively easily.

[0007]

However, since the output amplitude of the pyroelectric infrared detection element is very small, the amplification factor of the AC amplification circuit is conventionally 60 to 70 dB.
A high value was needed. That is, the FET in the element output stage is used as a source follower in order to eliminate or prevent malfunctions and the like due to fluctuations in the power supply voltage, and the power supply voltage is applied to the drain so that fluctuations in the output voltage do not occur. On the other hand, since the amplification factor of the source follower is 1 or less, the output amplitude becomes small and the amplification factor of the AC amplification circuit in the subsequent stage must be increased.

In order to increase the amplification factor of the AC amplifier circuit, a high resistance has to be used as the feedback resistor of the AC amplifier circuit. When a high resistance is used as the feedback resistance, the offset voltage generated by this high resistance and the bias current of the OP amplifier flowing through this resistance becomes large, which causes the stability of the AC amplification circuit to deteriorate. Furthermore, since the amplification factor is high, the AC amplifier circuit becomes sensitive to battery noise from the outside of the device, which may cause malfunction. It is necessary to add various circuits in order to prevent the occurrence of malfunction, and such measures against noise increase the price.

The present invention has been made to solve the above problems, and increases the amplification factor of an AC amplifier circuit by increasing the amplitude of a signal output from a pyroelectric infrared detecting element. It is an object of the present invention to provide an infrared detection device that is reduced in stability, has good stability, and is resistant to electromagnetic noise from the outside.

[0010]

In order to achieve such an object, the present invention has a power source for applying a voltage to a pyroelectric infrared detecting element, which has means for making the applied voltage a constant voltage. It is characterized in that the drain of the FET is connected to an AC amplifier circuit and also connected to a power source through a resistor.

[0011]

In the present invention, the method of connecting the FET and the AC amplifier circuit incorporated in the pyroelectric infrared detecting element is changed from the source follower to the source ground. As a result, the amplification factor of the FET increases and the amplitude of the output signal of the pyroelectric infrared detection element increases. As a result, the value of the feedback resistance of the AC amplifier circuit is reduced, the stability of the circuit is improved, and noise countermeasures are realized. Further, the power supply voltage of the FET is stabilized, and even if the drain potential changes due to the gate potential change, the FET operates stably because of the resistance.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. The same components as those in the conventional example shown in FIGS. 5 and 6 are designated by the same reference numerals and the description thereof will be omitted.

1 and 2 show the structure of an infrared detecting element according to an embodiment of the present invention. In particular, FIG. 1 shows the configuration of the pyroelectric infrared detection element and the AC amplification circuit in this embodiment, and FIG. 2 shows the overall circuit configuration.

First, as shown in FIG. 1, the pyroelectric infrared detection element 40 of the present embodiment is composed of two pyroelectric bodies 10, a resistance element 12 and a FET 14. The pyroelectric infrared detection element 40 is different from the pyroelectric infrared detection element 16 in the conventional example in that the source follower is used with the source grounded. That is, the source of the FET 14 is grounded via the resistor 22, and the drain is connected to the constant voltage circuit 44 via the resistor 42.

Further, the AC amplifier circuit 46 in this embodiment has substantially the same configuration as the AC amplifier circuit 18 of the conventional example shown in FIG. 5, except that the non-inverting input terminal of the OP amplifier 20 is connected to the AC amplifier circuit. The difference is that the input voltage is obtained from the drain of the FET and the connection end of the resistor 22 and the capacitor 24 is grounded.

The operation of this embodiment is shown in FIGS. First, when stepwise infrared rays as shown in FIG. 3 (a) are incident on the pyroelectric body 10 of the pyroelectric infrared detection element, the pyroelectric body 10 moves as shown in FIG. 3 (b). , Generate electric charges according to changes in the amount of incident infrared rays. The resistance element 12 converts this charge into a voltage, and the FET 14 converts this into impedance. Specifically, the pyroelectric body 1
The charge generated at 0 changes the gate voltage of the FET 14 (FIG. 3C), and as a result, the drain current of the FET 14 changes. When the drain current changes, the potential at the connection point between the drain and the resistor 42 changes. This potential becomes an input to the AC amplifier circuit 46. In this way, FE
Since T14 is used with the source grounded, by setting the value of the resistor 42 to an appropriate value,
It is possible to amplify the signal by about 14 dB.

As described above, the FET 14 has already set 10 to 10.
As a result of being amplified by about 14 dB, the amplification degree of the AC amplification circuit 46 can be lower than in the conventional case. That is, about 60-
When trying to secure the total amplification of about 70 dB,
The AC amplifier circuit 46 may have an amplification degree of about 60 dB at the maximum.

According to the results of experiments by the inventor, when the resistance value of the resistor 42 is 56 kΩ and the output voltage of the constant voltage circuit 44 is 3.0 V, the amplification degree in the FET 14 is 13.9 dB and the amplification degree in the AC amplification circuit 46 is It became 49.5 dB. Therefore, according to this embodiment, the amplification degree of the AC amplifier circuit 46 is reduced by about 14 to 20 dB as compared with the conventional example. That is, as shown in the comparison between FIG. 4B and FIG.
Even with the same gate voltage of ET14, a larger output signal can be obtained. As a result, the resistance value of the feedback resistor 28 in the AC amplifier circuit 46 can be set to 1/5 to 1/10 of that in the conventional case, the offset in the OP amplifier 20 is reduced, and the stability of the circuit is improved. Can be made

In this embodiment, the FET 14
A constant voltage circuit 44 is used to supply a power supply voltage to the above. In recent years, the constant voltage circuit 44 has become available as an IC at low cost. Therefore, it is relatively easy to manufacture the device of this embodiment at low cost.

[0020]

As described above, according to the present invention,
A FET incorporated in the pyroelectric infrared detection element while applying a constant power supply voltage to the pyroelectric infrared detection element
Since it is used with the source grounded, the amplification factor of the FET incorporated in the pyroelectric infrared detection element can reduce the amplification degree of the AC amplification circuit, and the stability and noise resistance of the circuit are improved. Further, since the value of the feedback resistance of the AC amplification circuit can be reduced, it becomes possible to configure the AC amplification circuit using an OP amplifier which has a characteristic lower than that of the conventional one but is inexpensive, and an inexpensive infrared detection device is provided. Will be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a pyroelectric infrared detection element and an AC amplification circuit in an infrared detection device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the overall circuit configuration of this embodiment.

FIG. 3 is a diagram showing the operation of the pyroelectric infrared detection element in this example, and FIG. 3 (a) shows incident infrared rays.
FIG. 3B is a waveform diagram showing the generated charge, and FIG. 3C is a waveform diagram showing the gate voltage.

FIG. 4 is a diagram showing an effect of this embodiment, and FIG.
Is the gate voltage, FIG. 4 (b) is the output signal in this embodiment, and FIG. 4 (c) is the output signal in the conventional example of FIG.
It is a waveform diagram respectively shown.

FIG. 5 is a circuit diagram showing a configuration of an infrared detection device according to a conventional example, particularly a pyroelectric infrared detection element and an AC amplification circuit thereof.

FIG. 6 is a block diagram showing the overall circuit configuration of this conventional example.

[Explanation of reference numerals] 10 Pyroelectric body 12 Resistance element 14 FET 20 OP amplifier 28 Feedback resistance 42 Resistance 44 Constant voltage circuit 46 AC amplification circuit

Claims (1)

[Claims] 1. A pyroelectric infrared detection element for detecting a change in an incident amount of infrared radiation, a power supply for applying a voltage to the pyroelectric infrared detection element, and a pyroelectric infrared detection element connected to a gate. An FET that impedance-converts the output of the infrared detection element, an AC amplifier circuit that amplifies the output of the FET,
In the infrared detection device including, the power supply has means for making the voltage applied to the pyroelectric infrared detection element a constant voltage, and the drain of the FET is connected to the AC amplification circuit and connected to the power supply via a resistor. An infrared detection device characterized by the above.